The invention relates to a steel with a high wear resistance, high hardness and good impact strength, utilizable for the manufacture of products, in the application of which at least some of said features are desirable, preferably for the manufacture of tools intended to be used at temperatures up to at least 500xc2x0 C. The invention also relates to the use of the steel, a product made of the steel and a method of producing the steel.
For moulding tools and machine components which are exposed to high mechanical and thermal fatigue stresses, such as moulding tools e.g. for extrusion, die-casting and for forging tools, valves and the like, hot-working steels or high-speed steels are generally used. Of the hot-working steels it is primarily steels of the type AISI H13 and of the high-speed steels mainly AISI M2 which are used. Both are conventional and have been known for more than 50 years. Many variations of H13 and M2 have also been proposed and used to a certain extent, but the classic H13 and M2 steels still predominate in their application areas.
It is an object of the invention to provide a steel with a better wear resistance than the most common type of hot-working steels, H13. Another object is that the steel shall have high hardness and toughness compared with dominant steel grades of the conventional type for hot-working applications. Yet another object is that the steel shall have high hot hardness and resistance down tempering at high temperature, something which is a typical characteristic of high-speed steels, which makes the material suitable as hot-working steel and as a substrate for coating using PVD technology. An object of the invention also in this regard, however, is that the steel shall have a lower content of expensive alloying components, such as tungsten and molybdenum, than conventional high-speed steels, such as high-speed steels of the M2 type. A further object of the invention is that the steel shall have good workability in the soft-annealed state of the steel and that it shall also be capable of being machined, e.g. ground, in the hardened state.
These and other objects can be achieved therein that the steel is produced powder-metallurgically, that it has a chemical composition as stated in appending claim 1 and that it contains 1.5-2.5 percent by volume of MC carbides, in which M consists essentially only of vanadium, said carbides being evenly distributed in the matrix of the steel.
The powder-metallurgical production of the steel can be carried out by applying known technology to produce steel, preferably by using the so-called ASP(copyright) process. This comprises the production of a steel melt with the chemical composition intended for the steel. Powder is produced from the melt in a known manner by gas-atomisation of a stream of molten metal, i.e. by deintegrating it into small drops by means of jets of inert gas, which are directed at the stream of molten metal, which drops are rapidly cooled so that they solidify to form powder particles during free fall through the inert gas. Following screening, the powder is inserted into capsules, which are cold-compacted and then exposed to hot isostatic compaction, so-called HIP-ing, at high temperature and high pressure to full density. HIP-ing is typically carried out at an isostatic pressure of 900-1100 bar and a temperature of 1000-1180xc2x0 C., preferably 1140-1160xc2x0 C.
With reference to the contents of the various alloying components in the steel, the following applies.
Vanadium shall be present in a content of at least 1.2% and max. 1.8% in order together with carbon to form 1.5-2.5 percent by volume of MC carbides in the steel. The powder-metallurgical production process creates the conditions for these carbides to acquire the form of small inclusions of essentially equal size with a typically round or rounded shape and even distribution in the matrix. The maximum size of the MC carbides, reckoned in the longest length of the inclusions, is 2.0 xcexcm. More precisely, at least 90% of the total carbide volume consists of MC carbides with a maximum size of 1.5 xcexcm, and more precisely these carbides have a size which is greater than 0.5 but less than 1.5 xcexcm. The MC carbides can also contain a small quantity of niobium. Preferably, however, the steel is not deliberately alloyed with niobium, in which case the niobium carbide element in the MC carbides can be disregarded. As well as carbon, a small quantity of nitrogen can also combine with vanadium to form the hard inclusions, which are here designated MC carbides. However, the nitrogen content in the steel is so small that the nitrogen component in the inclusions does not prompt the designation vanadium carbonitrides, but can be disregarded. The content of vanadium amounts preferably to 1.3-1.7%. The nominal vanadium content in the steel is 1.5%.
Carbon shall be present in the steel in a sufficient quantity to combine on the one hand with vanadium to form MC carbides in the above quantity, and on the other hand to be present dissolved in the matrix of the steel in a content of 0.4-0.5%. The total content of carbon in the steel shall therefore amount to 0.55-0.65%, preferably to 0.57-0.63%. The nominal carbon content is 0.60%.
Silicon shall be present in the steel in a minimum content of 0.7%, preferably at least 0.85%, to contribute to the hot hardness of the steel and its resistance to tempering during use. However, the content of silicon must not exceed 1.5%, preferably max. 1.2%.
Manganese is not a critical element in the steel according to the invention but is present in a quantity of between 0.2% and 1.0%, preferably in a content of between 0.2% and 0.5%.
After hardening and tempering, the steel according to the invention does not contain any notable content of chromium carbide, e.g. M7C3- or M23C6-carbides, which normally occur in hot-working steels. The steel according to the invention may therefore contain a max. of 5% chromium, preferably a max. of 4.5% chromium. However, chromium is in itself a desirable element in the steel and shall be present in a minimum content of 3.5%, preferably at least 3.7%, in order to contribute to the hardenability of the steel and together with molybdenum, tungsten and carbon to give the martensitic matrix of the steel in the hardened state the character of a high-speed steel, i.e. a good combination of hardness and toughness. The nominal chromium content is 4.0%.
Molybdenum and tungsten shall both be present in the steel, preferably in roughly equal amounts in order together with carbon and chromium to give the matrix of the steel its features just stated. Tungsten and molybdenum also contribute to counteract decarburization when they are correctly balanced relative to one another. Molybdenum and tungsten shall therefore each be present in a content of at least 1.5% and max. 2.5%, preferably in a content of between 1.7 and 2.3%. The nominal content is 2.0% for both molybdenum and tungsten.
Nitrogen is not added deliberately to the steel but can occur in a content of from 100 to 500 ppm.
Oxygen is an unavoidable impurity in the steel but can be tolerated owing to the powder-metallurgical production process of the steel in amounts up to 200 ppm.
Other impurities, such as sulphur and phosphorus, can occur and be tolerated in amounts, which are normal for hot-working steels and high-speed steels. This also applies to impurities in the form of metals, such as tin, copper and lead, which are not dissolved in the austenite in the austenitic state of the steel, and which are precipitated following solidification, as the austenite grains are formed at high temperature, said impurities being distributed over a large surface, as the austenite grain size is small, whereby concentrations of these impurities are countered, which renders the impurities harmless. However, the steel according to the invention typically does not contain impurity metals of the type tin, copper and lead in amounts of more than 0.10, 0.60 and 0.005% respectively and in total not more than a max. of 0.8% of said or other undesirable impurity metals.
The products for which the steel is intended to be used can be worked to near final shape, which can be carried out in a conventional manner, by means of cutting machining, e.g. milling, drilling, turning, grinding etc. or by means of spark machining in the soft-annealed state of the steel. In its soft-annealed state, the steel has a hardness of 230 HB max. (Brinell hardness), which can be obtained by soft-annealing of the steel at 850-900xc2x0 C. and then cooling to room temperature, with at least the cooling from the soft-annealing temperature down to 725xc2x0 C., and preferably down to at least 700xc2x0 C., being carried out as slow, controlled cooling at a cooling rate of 5-20xc2x0 C./h, preferably at a cooling rate of approx. 10xc2x0 C./h. Cooling to room temperature from at least 700xc2x0 C. or a lower temperature can take place by means of free cooling in air.
After hardening and tempering, the steel according to the invention has a hardness of 50-59 HRC (Rockwell hardness) and an impact strength corresponding to an absorbed impact energy of 150-300 Joule in an impact test using an un-notched test specimen with the dimensions 7xc3x9710xc3x9755 mm, and a structure of tempered martensite containing said MC carbides evenly distributed in the martensite, obtainable through hardening of the product from an austenitization temperature of between 950 and 1160xc2x0 C., cooling to room temperature and tempering at 540-580xc2x0 C. Depending on what the object produced from the steel is to be used for, i.e. the application range of the steel, an optimal hardness is selected in the hardness range 50-59 HRC. For hot-working applications, e.g. for hot-working rolls, forging tools and dies and other parts for the extrusion of aluminium, the optimum hardness range is between 52 and 58 HRC, taking the desired good impact strength into consideration. A hardness in said range can also be optimal for machine components intended to work at room temperature or at a temperature up to 500xc2x0 C., although hardnesses down to 50 HRC can also be acceptable for this type of products. The steel according to the invention can however also be used for cold-working tools and wear parts, in which case an optimal hardness can be 56-59 HRC, possibly at the expense of a certain reduction in impact strength at hardnesses up to 59 HRC. The desired hardness in said ranges is achieved by the choice of austenitization temperature in the range 950-1160xc2x0 C. according to the principle xe2x80x9cthe higher the austenitization temperature, the greater the hardnessxe2x80x9d, and vice-versa.
Further features and aspects of the invention are evident from the claims and from the following description of experiments carried out.